This invention relates to the grinding of cutting tools, and is particularly, but not exclusively, concerned with a method of operating a computer numerically controlled (CNC) grinding machine to product cutting tools with wavy cutting edges, and to such cutting tools produced by the method.
Rotary cutting tools generally have a series of helical blades. These blades are generally formed by two adjoining faces, a leading face (e.g. a flute surface) and a trailing face (e.g. a relief surface). The adjoining faces in turn form a crest with a sharp edge of generally smooth helical configuration. This edge is usually called the cutting edge. A particular type of milling cutter has helical blades with cutting edges of a wavy or sinusoidal configuration. Such a cutting edge configuration facilitates removal of material from a workpiece in small metal chips rather than in long lengths of swarf. Accordingly, cutting tools with a wavy cutting edge configuration are often referred to as xe2x80x9croughcutxe2x80x9d tools.
Roughcut tools usually have their blades formed of high speed steel or tungsten carbide. However, various problems have previously been experienced in the manufacture of such milling cutters with wavy cutting edges.
CNC grinding machines can be used to grind helical flutes and relief faces in a high speed steel workpiece to form the helical cutting edges. However, grinding wheels of CNC machines are usually of greater diameter than the width of the flutes, and this has hitherto precluded the exclusive use of grinding wheels to grind a complete milling cutter blade with cutting edges of wavy or sinusoidal configuration in a single setup from a solid blank. Thus, a separate setup on different machines is usually required to perform the machining operations necessary to generate the flute and relief surfaces of such blades. For instance, the relief surfaces are often ground on a mechanically controlled re-sharpening machine, while the wavy portion of the flute surface is machined on a milling machine by using a ball nose mill of smaller diameter than the width of the flutes between the helical leading and trailing edges. Such ball nose mills are not, however, suitable for tungsten carbide. When it is desired to have a blade, featuring such a cutting edge, formed of tungsten carbide, this is usually achieved by incorporating a tungsten carbide insert with the required wavy cutting edge into a tool formed of another material, but such a tool can be expensive to manufacture.
It is therefore desirable to provide an improved method of producing a cutting tool with wavy cutting edges using a CNC tool grinding machine.
According to one aspect of the invention, there is provided a method of operating a CNC grinding machine to grind a blade with a cutting edge of wavy configuration in a workpiece, the tool grinding machine having at least one rotatable grinding wheel movable relative to the workpiece, said method including:
programming the CNC machine with at least one path for the at least one grinding wheel, the at least one path being calculated to grind at least one surface of a blade with a generally smooth cutting edge in the workpiece;
programming the CNC machine with perturbations to said at least one path, the perturbations varying the movement of that least one grinding wheel in such a manner that the at least one grinding wheel is arranged to grind at least one surface of the blade with a cutting edge of a wavy configuration in the workpiece.
According to another aspect of the invention, there is provided a CNC grinding machine having at least one rotatable grinding wheel movable relative to a workpiece for grinding at least one blade in the workpiece, the machine being programmed with at least one path for the at least one grinding wheel which is calculated to grind at least one surface of a blade with a generally smooth cutting edge, wherein the machine is programmed with perturbations to said at least one path, the perturbations varying the movement of the at least one grinding wheel from said at least one path in such a manner that the at least one grinding wheel grinds a blade with a cutting edge of wavy configuration in the workpiece.
According to a further aspect of the invention, there is provided a computer program, wherein the set of wheel positions defining said path is generated from data representing geometry for the blade with a generally smooth cutting edge and data representing the shape of said at least one grinding wheel, and the path adjustment means calculates the perturbed wheel positions from the set of wheel positions defining said path and from data representing the perturbations for the wavy cutting edge.
Preferably, the computer program is recorded on a memory or data storage medium, such as a floppy disc, CD-ROM or the like, which may be supplied to a person having a programmable CNC tool grinding machine. It is, however, contemplated that the computer program could be downloaded by such a person over a computer network, such as the Internet.
The CNC machine preferably has a programmable control unit (PCU) into which data representing the geometry for the blade with a generally smooth cutting edge and the shape of the grinding wheel is entered, together with data representing the required perturbations for the wavy cutting edge.
The CNC machine may have a plurality of grinding wheels, in which case the machine is programmed with a plurality of paths for the grinding wheels for grinding the surfaces of the blade with the generally smooth cutting edge, and the machine is programmed with perturbations to said paths.
Each programmed path which is calculated to grind at least one of the surfaces of a blade with a generally smooth cutting edge is defined by a set of wheel positions for the at least one grinding wheel. Preferably, the CNC machine includes a wheel position generator for calculating the set of wheel positions defining the programmed path for grinding the generally smooth cutting edge, and a wheel position adjustment means for calculating perturbed wheel positions for the grinding wheel which are offset from said programmed path. Preferably, the wheel positions are offset in a linear or rotary direction, or combinations thereof, by definition of specific functions for offset magnitudes in said direction or directions such that, for each perturbed position, the grinding wheel contacts the workpiece at a single location of the perturbed cutting edge, or, for second or further relief facets, the perturbed leading edge of the relief facet, while avoiding overcutting or undercutting of the edge.
The leading edges of relief facets may also be represented by parametric curves with the path offset method above being applied in similar manner for grinding the relief facets with wavy leading edges.
In a particularly preferred embodiment, overcutting or undercutting is avoided for each perturbed grinding wheel position, by calculating the magnitude of any distance between the grinding wheel and the cutting edge in an overcut or undercut position, determining whether said magnitude is less than a threshold value and, if not, revising the perturbed wheel position directly or incrementally until it is less than said threshold value.
According to a further aspect of the invention, there is provided a cutting tool having at least one blade with a wavy cutting edge produced by the method or by a CNC machine or computer program in accordance with one of the preceding aspects of the invention.
Whilst the present invention is particularly applicable to grinding blades with wavy cutting edges in a cylindrical, cutting tool, it will be appreciated that it may be applied to grind such blades in cutting tools of other shapes, such as conical or toroidal shaped cutting tools or irregular shaped tools having a central longitudinal axis of rotational symmetry.
As used herein, the term wavy is used to refer to any perturbations having the general shape of a wave or serpentine. It includes within its scope symmetrical sinusoidal perturbations, piece-wise or asymmetrical sinusoidal perturbations, part circular scallops in a cutting edge and free-form wavy perturbations.
According to a particular preferred embodiment of the invention there is provided a cutting tool having a wavy cutting edge wherein the wavy cutting edge includes asymmetric or piece-wise sinusoidal perturbations to a generally smooth helical cutting edge, preferably, with alternate perturbations having different wavelengths.